1. Field of the Invention
The present invention relates to an electrodeionization (EDI) module. More particularly, it relates to an EDI module which has higher rates of ion removal and lower levels of scaling.
2. Description of the Related Art
Electrodeionization (EDI) is a process that removes ionizable species from liquids using electrically active media and an electrical potential between an anode and a cathode to influence ion transport. A typical system has at least one product channel through which liquid to be processed is flowed. At the outer edge of one side of the product channel is an anion permeable membrane which channel defines the outer limit of the product channel on that side. The opposite side of the product channel is defined by a cation permeable membrane. Waste channels are formed on the opposite sides of the membranes from the product channel. The anion side waste channel is formed between the anion permeable membrane and the anode that is spaced outwardly and apart from the anion membrane. Likewise the cation side waste channel is formed between the cation permeable membrane and the cathode.
The product channel is filled with a mixed bed of ion exchange resin ion exchange materials. These ion exchange materials are either anion specific or cation specific. Typically, the anion ion exchange materials are relatively strong in their selection for anions. These ion exchange materials are often referred to as Type I ion exchange materials. Likewise, the waste channels are filled with mixed beds of similar resin.
The liquid to be purified is flowed into the product channel while an electrical potential is applied to the system. The ion exchange materials in the product channel selectively cause the ions in the liquid to attach to the bead surfaces where they are transferred from bead to bead toward the electrode (anode or cathode) they favor. Once they pass through the ion selective membrane, they are passed to ion exchange materials in the waste channels. A liquid is also flowed through each waste channel that removes the ions from the ion exchange materials and carries them to waste.
In practice, the results have been less than completely satisfying. For one reason, each type of bead has a fixed charge and certain affinity for the selected ion. The efficiency with which the ion transport occurs is related to the field efficiency of the system and the binding constant of the resin relative to the selected ion.
The Type I resin used in the current system is a strong anion resin having a relatively high binding constant for certain ions such as bicarbonate (HCO3) (approximately 20) as compared to a hydroxyl ions(OH) (approximately 1.0).
With this stronger affinity for certain ions such as bicarbonate, the Type 1 resin doesn""t easily release the ion causing the ion transport to slow down. To obtain the same efficiency in the system over time as more and more bicarbonate is bound to the resin, one has to increase the power supplied to the system. In areas with high calcium and/or carbon dioxide levels, this may mean that the system needs a voltage increase in under a month and is susceptible to scaling which leads to deterioration of the system""s performance. One may use a water softener to reduce the levels of calcium and a degasser to reduce carbon dioxide entering the system, but thisonly delays the inevitable. Additionally, the need to use a water softener and/or degasser also adds to the cost of the system and the softening chemicals and adds to the amount of waste liquid generated by the system.
U.S. Pat. No. 5,868,915 suggests changing a minor amount of the Type I anionic resin in the product and/or waste channel to a weakly ionized type such those with weak base or a Type II resin (7% is the percentage cited in the patent.) Further, it suggests eliminating the use of ion exchange media in the waste channels and using a carbon based electrically conductive bead instead.
U.S. Pat. No. 5,858,191 is similar to U.S. Pat. No. 5,868,915 and adds the suggestion to alternate Type I with Type II anion resins and cationic resin and to use either only cationic resins or alternating layers of cationic and then anionic resins rather than mixed resin beds in the waste channels.
The current systems all have drawbacks. Scaling and the need for additional voltage as they age are the most common. The use of a water softener and/or degasser adds to the overall cost of the system for both hardware and the chemicals needed. Additionally, it leads to a higher amount of waste liquid generated by the system. Moreover, to the inventors"" knowledge, systems based on U.S. Pat. Nos. 5,868,915 and 5,858,191 have never been commercialized.
The present invention overcomes the problems of the current system by providing a system that resists scaling and doesn""t typically require an increase in voltage as it ages. To the extent that a system does require the use of increased voltage, the time by which it is needed is far longer than that accomplished with today""s systems.
An electrodeionization module is formed having a product channel where the outer sides of the product channel is bordered on each side by an ion permeable membrane, and said membranes define the inner boundary of a first and second waste channel formed outside of the product channel so as to surround it. An anode is located on one side of the first waste channel farthest away from the product channel and the waste channel is defined between the anode an the first ion permeable membrane. The second waste channel is defined between the second membrane and the cathode placed on the side opposite that of the anode beyond the product and waste channels. The product channel is filled with a bed of mixed ion exchange materials of both anion and cation selective types. The waste channels are filled with mixed ion exchange materials wherein the anion ion exchange materials used has a low affinity for the specie or species desired to be captured. Alternatively, it, if desired, may be blended with some amount of Type I ion exchange materials. The membranes are ion selective, the first being anion permeable, the second being cation permeable. Preferably, the membranes contain an ion exchange material to speed the transfer of ions across them. More preferably, the anion permeable membrane contains anion exchange materials that have a low affinity for the selected specie or species for retention.
It is an object of the present invention to provide a system for the electrodeionization of liquids comprising a cathode, an anode spaced apart from the cathode, an electrical potential supplied between the cathode and anode, an anion permeable membrane spaced apart from the anode and defining a first waste channel between the anion permeable membrane and the anode, a cation permeable membrane spaced apart from the cathode and defining a second waste channel between the cation permeable membrane and the cathode, the anion permeable membrane and the cation permeable membrane being adjacent but spaced apart from each other so as to define a product channel, the product channel containing mixed ion exchange materials, the materials being anion selective and cation selective, the first and second waste channels containing a mixed ion exchange materials, the ion exchange materials comprising cation selective ion exchange materials and anion selective ion exchange materials, wherein the anion selective ion exchange materials are selected from the group consisting of anion ion exchange materials having a relatively low affinity for the selected anion (s) of interest and is typically selected from those anion exchange materials which have weakly basic groups, Type II functional groups and mixtures thereof.
It is a further object of the present invention to provide an EDI module comprising one or more product channels, one or more waste channels, an anode, a cathode and mixed ion exchange materials wherein the mixed ion exchange materials in the one or more waste channels are formed of cationic ion exchange materials and anionic ion exchange materials having relatively low affinity for selected specie(s) selected from the group consisting of anion ion exchange materials having weakly base groups, Type II functional groups and mixtures thereof.
It is another object of the present invention to provide a system for the electrodeionization of liquids comprising a cathode, an anode spaced apart from the cathode, an electrical potential supplied between the cathode and anode, an anion permeable membrane spaced apart from the anode and defining a first waste channel between the anion permeable membrane and the anode, a cation permeable membrane spaced apart from the cathode and defining a second waste channel between the cation permeable membrane and the cathode, the anion permeable membrane and the cation permeable membrane being adjacent but spaced apart ion exchange materials from each other so as to define a product channel, the product channel containing mixed ion exchange materials, the ion exchange materials being anion selective and cation selective, the first and second waste channels containing mixed ion exchange materials, the materials comprising cation selective ion exchange materials and anion selective ion exchange materials, wherein the anion selective ion exchange materials are selected from the group consistin~of anion ion exchange materials having relatively low affinity for the selected specie(s) the resins being selected from resins with weakly basic groups, materials having Type II functional groups and mixtures thereof and the anionic membrane contains an anion ion exchange material having a relatively low affinity for the selected specie(s).